This invention concerns railway trucks generally and apparatus for controlling truck hunting responses by resisting the relative rotational impetus of a truck with respect to a car body supported thereby.
In conventional railway trucks such as three-piece trucks, hunting is an undesirable mode of truck movement along the railway track rails which is characterized by unstable cyclic yawing and lateral translation of the truck as it follows the truck wheelsets in the travel of their sinusoidal path of movement along the rails. As is well known, the truck wheelsets commonly will travel such a sinusoidal path in response to the contact forces developed by the interaction of the conical wheels with the cylindrical rail heads. A truck wheelset traveling its sinusoidal path moves transversely in cyclic fashion with respect to the rails. In doing so the wheelset both traverses laterally and yaws in a horizontal plane with respect to the rails.
With the advent of the roller bearing for railway trucks, lateral wheelset freedom with respect to the truck side frames was essentially eliminated and the side frames thus were constrained to follow the wheelsets in their cyclic lateral traversing and yawing movements. In response to this constraint the truck may yaw cyclically about a vertical axis with respect to the car body supported thereon and/or translate laterally with respect to the rails.
When such responses occur above a given critical rail car speed, the truck movements can degenerate into self-excited oscillation of the truck which is known as truck hunting. This can be destructive to the truck wheels and the rails as well as to other truck components, the car body and its lading due to the large magnitude impact loads sustained by all of these components. In the extreme, truck hunting can precipitate catastrophic failure of various affected components and result in derailment.
The art has continually sought to reduce the incidence of truck hunting by various means including the use of fluid and friction damping to dissipate the energy input which drives the hunting responses. For example, it is known that a controlled resistance to the relative rotational movement between a truck and a car body supported thereby can be effective to restrain truck hunting. One commonplace means for providing such resistance to relative truck rotation, the constant contact side bearing, typically includes a sliding contact member that is maintained in biased frictional engagement with a car body wear plate. Truck yaw with respect to the car body thus results in shearing movement between the side bearing contact member and the car body wear plate with resultant wheel-rail creep damping as well as sliding friction that dissipates a portion of the energy that drives the truck yawing movement.
Since a truck must yaw with respect to the car body supported thereon in normal operation, such as when the car traverses a track curve entry or exit spiral, it is preferable that the control of truck yaw with respect to the car body be maintained only for higher velocity rotational movements which would be characteristic of destructive truck hunting responses, and not for lower velocity truck yawing such as that encountered when the car negotiates a track curve. For this purpose, a yaw resistance of a magnitude that is related to truck yaw movement velocity is a desirable alternative or adjunct to a constant contact side bearing.
The present invention contemplates such an alternative in the form of a hydraulic yaw damper connected between a railway truck, for example the truck bolster, and a railway car body supported thereon. Although hydraulic dampers, and in particular elongated, selectively extensible piston and cylinder hydraulic dampers, are known in the railway arts, none so far as we are aware provides the improved structure or the method disclosed hereinbelow.
For example, U.S. Pat. No. 4,300,454 discloses a self-steering damping railway truck which includes a hydraulic damper for damping relative movement between the truck wheelsets. U.S. Pat. No. 4,134,343 discloses a railway truck having a hydraulic piston and cylinder assembly connected by links to brake shoes and utilized for application of braking force. U.S. Pat. No. 4,109,767 discloses a valve structure for a double acting shock absorber with a compression valve being mounted within a hollow valve stem of a replenishing valve. Other patents of pertinence to hydraulic shock absorbers and similar structures include U.S. Pat. Nos. 5,070,697, 5,044,474, 4,673,068, 5,082,309, 4,946,354, 4,940,106, 4,899,855, 4,580,710, 4,513,899, 4,280,601, 4,262,922, 4,178,030, 4,113,111, 4,108,010, 4,105,193, 4,090,723 and 4,023,689.